Kressel et al in U.S. Pat. No. 3,747,016 issued July 17, 1973 and entitled "SEMICONDUCTOR INJECTION LASER", have disclosed a light emitting device composed of an active layer interposed between transparent confining layers having a lower index of refraction than that of the active layer. Typically this device is fabricated with a GaAs substrate and active region and AlGaAs upper and lower confining layers. A GaAs capping layer overlies the upper confining layer for contacting purposes. Ettenburg et al, in IEEE Journal of Quantum Electronics QE-12, 360 (1976), have disclosed that a light beam propagating in the active region spreads into the transparent confining layers as the active region is narrowed to less than about 0.1 micrometer in the direction perpendicular to the plane of the layers. Since the crossection of the beam at the end surface is then larger, the angular divergence of the emitted light beam from diffraction effects is then reduced.
Antypas in U.S. Pat. No. 3,982,261 issued Sept. 21, 1976 and entitled "EPITAXIAL INDIUM-GALLIUM-ARSENIDE PHOSPHIDE LAYER ON LATTICE-MATCHED INDIUM-PHOSPHIDE SUBSTRATE AND DEVICES" has disclosed that a light emitting device can be fabricated using an InGaAsP alloy as the active layer and InP as the confining layers. This device emits radiation at wavelengths between about 1.2 micrometers and about 1.7 micrometers depending upon the relative concentration of the constituents of the InGaAsP alloy while maintaining the lattice match to the InP layers. The device is typically composed of an InP substrate, an InP lower confining layer, an InGaAsP active layer, an InP upper confining layer, and an InGaAsP capping layer for contacting purposes.
We have found that, as the thickness of the active layer is reduced below about 0.1 micrometer in this device, the output beam quality deteriorates producing an output beam having an asymmetric or multipeaked intensity distribution. This results in a reduced efficiency of coupling of light emitted by the device into an optical fiber.